1. Field of the Invention
This invention relates generally to a parking gear mounting structure for an automatic transmission and; more specifically, to a parking gear mounting structure that can provide a precise centering, with respect to an axis of an output shaft of the transmission, of a pinion carrier having a parking gear in a fixed relationship thereto, so as to securely prevent a radial displacement of the pinion carrier when the parking gear engages with a parking pawl provided at a side of a transmission case.
2. Description of the Background Art
There is known a parking gear mounting structure for an automatic transmission, such as disclosed in Second Japanese Patent Publication No. 56-44304.
In this known structure, a parking gear is mounted to a reduction gear unit of a sub-transmission having a final output shaft, which sub-transmission is mounted at an output side of a main transmission, for providing a compact sub-transmission structure. Specifically, the parking gear is integrally formed with a pinion carrier of the reduction gear unit and is engageable with a parking pawl provided at a side of a transmission case for locking the final output shaft to the transmission case when the transmission is in a parking position. The pinion carrier and the final output shaft are connected to each other by a spline connection. Specifically, the pinion carrier has a boss portion which receives the final output shaft therethrough. An inner periphery of the pinion carrier boss portion is formed with a plurality of radially inward and outward protruding portions with respect to an axis of the final output shaft, and an outer periphery of the final output shaft is also formed with a plurality of corresponding radially outward and inward protruding portions for engagement with the corresponding radially inward and outward protruding portions of the pinion carrier boss portion. Each radially inward or outward protruding portion extends in a direction along the axis of the final output shaft so as to transmit the driving power from the engine to the final output shaft through the pinion carrier. Accordingly, in this known structure, a positioning of the pinion carrier in a radial direction with respect to the axis of the final output shaft is performed substantially only by the spline connection between the pinion carrier boss portion and the final output shaft.
The final output shaft has a forward tip portion of a smaller diameter which is coaxially inserted into an axial bore formed at a rearward end portion of an intermediate output shaft of the main transmission and is supported by a bearing disposed between an outer periphery of the smaller diameter forward tip portion of the final output shaft and an inner periphery of the rearward end portion of the intermediate output shaft of the main transmission. On an outer periphery of the rearward end portion of the intermediate output shaft is mounted a connecting member by a spline connection which is substantially the same in structure as described above. The connecting member works to mechanically connect the intermediate output shaft of the main transmission to the reduction gear unit of the sub-transmission. Further, another bearing is disposed between the rearward side of the connecting member and the forward side of the pinion carrier so as to restrain displacement of the pinion carrier in a direction along the axis of the final output shaft or that of the coaxial intermediate transmission output shaft.
In this known parking gear mounting structure, however, as described above, the radial positioning or the centering of the pinion carrier having the integral parking gear with respect to the axis of the final output shaft is performed substantially solely by the spline connection as mentioned above. Because substantial backlash is provided in the spline connection, the pinion carrier is forced to move radially relative to the final output shaft due to a biasing force applied to the parking gear when the parking gear engages with the parking pawl, depending on a magnitude of the backlash in the spline connection. As a result, the engagement between the parking gear and the parking pawl is likely to become unreliable and further, an unexpected excessive force is likely to exerted onto pinions supported by the pinion carrier to deteriorate durability of the reduction gear unit.
Further, the forward tip portion of the final output shaft has to be formed smaller in diameter to be inserted into the axial bore of the intermediate output shaft, and the wall of the rearward end portion of the intermediate output shaft defining the axial bore has to be formed smaller in thickness due to the spline connection with the connecting member. Thus, structural strength of the associated elements for supporting the pinion carrier becomes inevitably less, which also impairs the durability of the associated elements.